The present invention relates to ink jet printing and, more particularly, to an ink jet printer in which printer operation and reliability at start up and shut down are enhanced. A number of problems are encountered at the initiation of operation of an ink jet printer and at termination of printer operation.
Typically, ink jet printers include a print head defining a fluid manifold or reservoir to which electrically conductive ink is supplied under pressure. A plurality of orifices are defined by an orifice plate, with each of the orifices communicating with the fluid reservoir. The orifices may typically be positioned in a pair of parallel rows, as illustrated in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis. Ink is forced under pressure through the orifices and emerges as a plurality of fluid filaments. Varicosities are generated in the fluid filaments by mechanical stimulation of the orifice plate or by generating pressure waves which travel through the ink in the fluid reservoir. The filaments are thereby caused to break up into streams of ink drops of substantially uniform size and spacing.
Charge electrodes are positioned beneath the orifice plate and adjacent the filament tips. Charge potentials selectively applied to the charge electrodes induce corresponding charges on the drops formed from the filament tips. The charged and uncharged drops then pass downwardly through a deflection field, with the charged drops deflected into a first set of trajectories and the uncharged drops unaffected passing through the field. Drop catchers are positioned outwardly of the rows of jet drop streams and in some printers may cooperate with with a deflection electrode, extending between the rows of jet drop streams, to produce deflection fields when a deflection potential is applied between the deflection electrode and the catchers. Alternatively, the deflection electrode may be eliminated with a deflection potential being placed across the opposing catchers to produce a deflection field. With such an arrangement, drops in each of the rows of jet drop streams are charged to opposite polarities to effect outward deflection of charged drops toward the catchers. Drops may be charged binarily or to a plurality of charge levels, as required by the particular printer configuration. Drops not sufficiently deflected to strike a catcher pass through the deflection field and are deposited upon a print receiving medium.
At start up of such a printer, the fluid flow through the orifices and the formation of drops from the filaments are extremely irregular and unpredictable. Exceptionally large drops of ink may be formed from the filaments and the trajectories of such drops are relatively uncontrolled. As a consequence, large amounts of ink may be deposited upon the charge electrodes and upon the deflection field electrode structure. Such large drops tend to short out the charge electrodes and deflection electrode structure, and may also interfere with the trajectories of the jets once stable jet operation is obtained. The large drops cannot be predictably deflected toward the catchers because generally it is not possible to apply a significant charge to the irregularly sized drops. Even if a charge were to be induced in such drops, the charged to mass ratio of the drops may be so small as to preclude effective deflection of the drops. Similar problems are encountered at shut down of the printer as the pressure of the ink is reduced and fluid flow through the orifices of the orifice plate is terminated.
Several approaches have been taken in order to overcome the problems presented by jet instability at start up and shut down of an ink jet printer. As shown in U.S. Pat. No. 4, 081,804, issued Mar. 28, 1978, to Van Breemen et al, a print head has been mounted over a drip pan at start up to collect drops formed from the fluid filaments until the jets become stable. A print receiving medium is thereafter passed beneath the print head, above the drip pan, and printing is initiated. The Van Breemen et al patent also discloses pivotal mounting arrangements for the catchers in which the catchers may be pivoted downward and outwardly from the print head to permit inspection of the charge electrode structure.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 1, June 1977, pp, 33 and 34, a charge electrode structure is shown in which a notched charge electrode plate is pivoted or, alternatively, translated into position adjacent the jet drop streams after start up to reduce wetting of the charge electrodes by the unstable jet drop streams.
IBM Technical Disclosure Bulletin, Vol. 19, No. 8, January 1977, pp, 3216 and 3217, discloses an ink jet printer in which a pair of charge electrode plates are moved laterally into and out of operating positions after start up and prior to shutdown, respectively. Additionally, a pair of catchers, positioned outwardly of the two parallel rows of jet drop during operation of the printer, are moved laterally together into contact at start up and shut down to prevent splatter of the ink on the print receiving medium. All of the drops are charged and deflected before the catchers are moved apart at start up, and before catchers are moved together at shut down, to prevent the drops produced by the unstable jets from reaching the paper beneath the print head or the paper support structure. Since the catchers in this print head are moved together beneath the pair of rows of jet drop streams which then strike the upper surfaces of the catchers, it is necessary that the upper surfaces of the catchers be formed of a porous material to ingest adequately the substantial flow of ink which is produced by the jets. Additionally, since the catchers are maintained in contact until after charging and deflection of the drop streams is initiated at start up, additional deflection electrodes are necessary in order to provide a deflection field and the catchers themselves may not be used to provide such a field.
Accordingly, there is a need for a simple, reliable arrangement for an ink jet printer which ensures that drops formed from unstable jets at start up and shut down are caught and, further, that the charge electrode structure and other printer elements are not contaminated by drops produced from the unstable jets.